1. Field of the Invention
This invention relates to the press bending of heat softened sheets and in particular to a vacuum mold for press bending heat softened apertured glass sheets.
2. Technical Considerations
Shaped and tempered glass sheets are widely used as side windows or rear windows in vehicles such as automobiles or the like. To be suitable for such applications, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frame defining the window opening into which the windows are installed. It is also important that these windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with clear viewing therethrough in the viewing area.
In a typical commercial operation for the production of shaped glass sheets, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace where the glass sheet is one of a series of sheets that are heated to the softening temperature of the glass and then along an extension of the path into a shaping station where each glass sheet is engaged by and pressed between a pair of shaping molds. The upper shaping mold may be a vacuum mold with an apertured sheet engaging surface so that a vacuum can be drawn through the mold to hold the heat softened sheet by suction. After shaping, a transfer and tempering ring having an outline shape conforming to that desired for the glass sheet slightly inboard of its perimeter moves upstream into a position below the shaping mold. The vacuum releases and deposits the glass sheet onto the tempering ring. The tempering ring supports the peripheral edge of the glass sheet while it conveys the shaped glass sheet into a cooling station where it is cooled in a controlled manner to a temperature below the annealing range of the glass.
Apertured glass sheets may cause problems when formed by press bending and in particular, when pressed between a pair of high heat capacity bending molds. The absence of glass due to the apertures in the glass provides a void that is not contacted by a corresponding portion of the mold surface. It is believed that this non-contacted portion of the mold tends to draw additional heat away from the perimeter portion of the aperture. As a result, the temperature of the glass around the periphery of the aperture is less than that of the remainder of the glass. With such an arrangement, when the glass is cooled for tempering, the periphery of the aperture develops a lesser temper than the remaining portions and this increases the occurrence of venting and breakage at the apertures.
U.S. Pat. No. 4,319,907 to Pike teaches a solution to this problem for shaping apertured glass sheets in a vertical press bending operation wherein the heat transfer capacity of the mold in the vicinity of the apertures is reduced. In particular, the mold surface is provided with holes aligned with the apertures in the glass, these holes in the mold surface being slightly larger than the glass apertures in the glass. Since that portion of the mold surface that would have been adjacent the apertures in the glass has been removed, there is no structure that will draw additional heat from the periphery of the aperture so as to effect its temper upon cooling. The resulting heat transfer at the glass sheet aperture during pressing improves the stress pattern around the peripheral portions of the glass sheet apertures.
If the arrangement of U.S. Pat. No. 4,319,907 were used in a vacuum mold as is typically employed in a horizontal press bending operation, the large holes in the press face would require a large volume of air to be drawn through the vacuum mold in order to hold the bent glass sheet thereagainst. In addition, the volume of air drawn through the hole in the mold may adversely cool the peripheral edge of the glass aperture prior to tempering, thus reducing its tempered properties and resulting in additional venting and breakage. Furthermore, the increased volume of air drawn through the hole necessary to hold the glass sheet against the mold surface may also deform the glass sheet aperture about its periphery.
It would be advantageous to have a vacuum mold capable of shaping apertured glass sheets without adversely affecting the peripheral edge of the apertures so as to reduce venting and breakage and maintain the proper bent shape at the glass sheet aperture.